1. Field of the Invention
This invention relates to improved resorcinolic resins having a low free resorcinol content, and methods of making and using the same. These resins are particularly useful as bonding and stiffening agents in the production of rubber compositions, imparting improved physical and mechanical properties such as low volatility, reduced fuming in rubber compounding, and improved adhesion properties of vulcanized rubber and rubber composites.
2. Background Information
The performance of rubber composites in articles such as tires, belts and hose products depends on numerous factors including the quality of adhesion between the rubber and reinforcing materials, and the mechanical properties of the cured rubber matrix in contact with the reinforcing materials.
The conventional method of optimizing the adhesion of the rubber to the reinforcement entails compounding into the rubber before vulcanization a two-part adhesive system that includes a methylene donor and a methylene acceptor. The methylene donor generates methylene or methylol groups capable of reacting or crosslinking with the methylene acceptor, which interacts with the rubber and reinforcing material with a resultant increase in adhesion of the rubber to the reinforcing material. Because the methylene donor and the methylene acceptor are compounded into the rubber, they can have a significant effect on the properties of the final rubber product.
Dihydric phenols, polyhydric phenols and phenolic novolak resins have long been used in the rubber industry as methylene acceptors. For example, resorcinol is a widely used dihydric phenol methylene acceptor for bonding rubber to the reinforcing materials. Resorcinolic methylene acceptors can present processing problems, however, by generating volatiles such as free resorcinol at Banbury temperatures.
The most commonly used methylene donors include, for example, hexamethylenetetramine and various methylol or methoxymethyl melamines. Other methylene donors are described in U.S. Pat. No. 3,751,331.
The in situ resin formed by the reaction of the resorcinolic methylene acceptor and the methylene donor promotes adhesion between the rubber and reinforcing materials which include, for example, steel, glass and organic filaments, fibers, cords and fabrics. In addition, the resorcinolic resin also provides rubber vulcanizates with improved hardness and dynamic stiffness properties. These compounding additives also act as thermosetting plasticizers, providing easier processibility, and excellent extrusions for the rubber compounds. The disadvantage, however, of these resorcinolic compounding additives is that they can generate an undesirable amount of fuming in Banbury mixing and calendaring operations.
U.S. Pat. No. 4,889,891 discloses alkyl substituted resorcinolic novolak resins as suitable methylene acceptors for vulcanizable rubber compositions.
U.S. Pat. No. 4,892,908 discloses the use of keto derivatives of resorcinol, such as benzoyl resorcinol, as methylene acceptors in vulcanizable rubber compositions.
U.S. Pat. No. 4,605,696 discloses use of monoesters of resorcinol, including resorcinol monobenzoate and resorcinol monorosinate, in rubber compositions.
U.S. Pat. No. 5,021,522 discloses aralkyl substituted resorcinolic novolak resins including a styrene substituted resorcinol formaldehyde resin.
U.S. Pat. No. 5,030,692 discloses alkylphenol modified resorcinolic novolak resins.
U.S. Pat. No. 4,731,430 discloses phenol resins modified with compounds possessing amide and/or imide groups that are useful as crosslinking agents and in the preparation of adhesives and rubber assistants.
U.S. Pat. No. 4,990,364 describes a process for producing porous phenolic resin fibers including the step of thermally decomposing a graft polymer containing a vinyl group.
U.S. Pat. No. 5,244,725 discloses a vulcanizable rubber composition that includes a rubber component, a methylene donor and a methylene acceptor.
U.S. Pat. No. 4,476,262 discloses a water dilutable resinous product prepared by reacting together a resin, an aldehyde and a sulfurous or organic acid.
There remains, however, a very real and substantial need for further improved bonding additive resins that may be used as methylene acceptors and that, in combination with suitable methylene donors, have enhanced curing properties, mechanical properties, and improved bonding and stiffening properties in vulcanizable rubber compositions. In addition, there remains a very real and substantial need for non-volatile resorcinol resins.